Axial flow compressors are commonly used in bypass-type jet engines to supply compressed air to the combustion chamber. These compressors are typically multi-stage units wherein each stage consists of a bank or row of rotor blades connected to the rotor disk and an adjacent row of stator vanes mounted to the compressor casing. The several stages of the compressor operate in series to gradually increase the pressure of the airflow therethrough in an aft direction for injection into the combustion chamber.
In bypass-type jet engines, only a portion of the air entering the forward end of the compressor passes through its high pressure stages. The rest of the air is ducted around the high pressure stages of the compressor and then later mixed in the exhaust system with the high pressure air before passing to the propelling nozzle of the jet engine.
In order to improve the aerodynamic performance of axial flow compressors for bypass-type jet engines, a flow splitter is sometimes mounted to the airfoil of the rotor blades in a low pressure stage of the compressor intermediate the root and tip of the airfoil. The purpose of the splitter is to divert the airflow at the intake or forward end of the compressor into a low pressure, bypass air stream which is ducted around the high pressure stages of the compressor, and a high pressure, inner air stream which passes into the high pressure stages of the compressor. This split or stratification of the air flow can increase the aerodynamic performance of the jet engine and thus lower fuel consumption.
In the prior art, blade splitters mounted to the airfoil of the rotor blades in the low pressure stages of a compressor have been "part span" shrouds, i.e., short, arcuate-shaped sections of metal alloys welded or otherwise permanently affixed to the airfoil of each rotor blade in the low pressure stage. The ends of the part span shroud of one airfoil abut the ends of the part span shroud of adjacent airfoils so that together the shrouds form an essentially continuous, annular-shaped ring around the entire rotor stage. The abutting ends of the part span shrouds are designed to create a seal therebetween to limit leakage of air between the inner and outer flow paths formed by the shrouds.
The stator vanes in the low pressure stage of the compressor adjacent to the rotor blades are also provided with shrouds similar in construction to those of the rotor blades. Both the part span shrouds of the rotor blades and the shrouds of the stator vanes are formed to engage one another along their facing, circumferential edges to create a seal therebetween. This circumferential seal between the rotating rotor blades and fixed stator vanes is intended to further limit the leakage of air between the inner air flow path and outer or bypass air flow path through the compressor.
Despite their aerodynamic advantages, part span shrouds of the type described above have several mechanical disadvantages. Each part span shroud is essentially deadweight on the airfoil and centrifugal loading produced by rotation of the rotor blades at high speeds induces a correspondingly high bending stress at the shroud-airfoil interface causing high local airfoil stresses. In turn, the centrifugal loads produced by the shrouds are transferred through the airfoils directly to the rotor disk which increases the overall weight of the rotor structure.
The high centrifugal forces applied to the shrouds by rotation of the rotor produces high bending stresses in the shrouds themselves and thus causes relatively large shroud deflections. Relative movement or deflection of the shroud mounted on one rotor blade airfoil with respect to the shroud mounted on an adjacent rotor blade airfoil allows air to leak therebetween. This diminishes the aerodynamic performance, i.e., splitting of the air incoming to the compressor into a low pressure bypass or outer flow and a high pressure or inner flow into the high pressure stages of the compressor.
Leakage of air between the high pressure air flow path through the compressor and the low pressure bypass air flow path can also occur along the contacting circumferential edges of the part span shrouds mounted to the row of rotor blades and the mating shrouds mounted to the adjacent row of stator vanes in the low pressure stage of the compressor. Such relative deflection is typically produced by centrifugal loading and airfoil untwist, i.e., a tendency of the airfoil to straighten under high centrifugal load. This airfoil untwist can cause the contacting shroud surfaces of the rotor blades and stator vanes to "lock up" and hence rub against one another causing additional shroud stresses and possible surface fatigue at the contacting surfaces of the shrouds.